Method and apparatus for activating a high frequency clock following a sleep mode within a mobile station operating in a slotted paging mode

ABSTRACT

A technique for activating an active-mode high frequency clock following a sleep period for use within a mobile station wherein selected components of the mobile station operate using a low power, low frequency sleep-mode clock during the sleep period and the faster high frequency active-mode clock during non-sleep periods. In one embodiment, the technique is implemented by a device having a wake-up estimation unit for estimating a wake up time using the sleep-mode clock and a frequency drift compensation unit for compensating for any error in the estimated wake up time caused by frequency drift in the sleep-mode clock. An off-set time compensation unit is also provided for compensating for a lack of precision in the low frequency sleep-mode clock resulting in a possible error in the estimated wake up time. The lack of precision can result in an initial timing off-set error at the beginning of the sleep period and an final timing off-set error at the end of the sleep period. Both the frequency drift compensation unit and the off-set time compensation unit employ a high frequency transition-mode clock signal for use in calculating the time required to adjust the wake-up time. The transition-mode clock, which may have the same frequency as the active-mode clock, is employed only at the beginning and end of the sleep period and is deactivated throughout most of the sleep period to reduce power consumption.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The invention generally relates to mobile communication systems and inparticular to techniques for activating a high frequency clock followinga sleep period within a mobile station of a mobile communications systememploying slotted paging.

II. Description of the Related Art

Certain state of the art wireless communication systems, such as CodeDivision Multiple Access (CDMA) Systems, employ slotted paging to allowmobile stations to conserve battery power. In a slotted paging mode,paging signals are transmitted from a base station to particular mobilestations only within assigned paging slots separated by predeterminedintervals of time. Accordingly, each individual mobile station mayremain within a sleep mode during the period of time between consecutivepaging slots without risk of missed paging signals. Whether anyparticular mobile station can switch from an active-mode to a sleep modedepends, however, upon whether the mobile station is currently engagedin any user activity such as processing input commands entered by theuser or processing a telephonic communication on behalf of the user.Assuming that the mobile station is not currently engaged in anyprocessing on behalf of the user, the mobile station automaticallypowers down selected internal compounds during each period of timebetween consecutive slots. One example of a slotted paging system isdisclosed in U.S. Pat. No. 5,392,287, entitled “Apparatus and Method forReducing Power Consumption in a Mobile Receiver”, issued Feb. 21, 1995,assigned to the assignee of the present invention and incorporated byreference herein.

Thus, within the slotted paging mode, a mobile station reduces powerconsumption by disconnecting power from selected internal componentsduring a sleep period between consecutive slots. However, even duringthe sleep period, the mobile station must reliably track the amount ofelapsed time to determine when the next slot occurs to permit receivecomponents of the mobile station to power up in time to receive anypaging signals transmitted to the mobile station within the slot. Onesolution to this problem is to operate a high frequency clock throughoutthe sleep period and to track the amount of elapsed time using the highfrequency clock. This solution allows the sleep period to be veryprecisely tracked. However, considerable power is consumed operating thehigh frequency clock and optimal power savings therefore are notachieved during the sleep period.

Hence, it would be desirable to instead employ a low frequency, lowpower clock during the sleep period to reduce power consumption.However, clock signals provided by low frequency, low power clockstypically suffer from considerable frequency drift such that the amountof elapsed time during the sleep period cannot be precisely determinedby counting cycles of the low power, low frequency clock. Frequencydrift within a mobile station can be particularly significant if thereare temperature variations within the mobile station caused by, forexample, heat generated by the operation of components of the mobilestation or by changes in ambient conditions. For example, during anextended telephone call, internal components of the mobile station mayheat to 87 degrees Celsius. During an extended period of inactivitybetween telephone calls, the internal components may cool to an ambienttemperature of, perhaps, 25 degrees Celsius. Moreover, if the userplaces the mobile telephone in either a very hot or very cold location,corresponding temperature changes within the mobile station may occur.Typical low power, low frequency clock signal generators are affectedsignificantly by even slight changes in temperature and are even morestrongly affected by the wide variations in temperature that can occurin a mobile telephone. Indeed, the amount of frequency drift within atypical low power, low frequency clock signal used in a mobile stationmay be so great that, if used by itself to calculate elapsed time withinthe sleep period, there is a significant risk that the mobile stationwill not be reactivated in time to power up components to detect apaging signal transmitted within a next paging slot. Accordingly,important paging signals maybe missed, possibly resulting in missedphone calls and the like. Thus the timing accuracy provided by a lowfrequency, low power clock signal is typically poor.

Another significant problem with using low frequency clock signals totrack elapsed time within a sleep period is the relative lack ofprecision provided by the low frequency clock. The lack of precision canresult in a considerable off-set between the initiation of the sleepperiod and a first counted cycle of the low frequency clock signal andalso a considerable off-set between a last counted cycle of the lowfrequency clock and the actual end of the sleep period. Morespecifically, a counter is typically employed to count either risingedges or falling edges of the low frequency clock signal to trackelapsed time within the sleep period and, once the number of cycles ofthe low frequency clock corresponding to the length of the sleep periodhave been counted, the high frequency clock is then re-activated.However, nearly an entire cycle of the low frequency clock may elapsebetween the beginning at the sleep period and the first edge of thelow-frequency clock signal detected by the counter. The initial off-setcan have a duration anywhere from zero to one full cycle of the lowfrequency clock or, in some systems, possibly even more. Withconventional systems, it is not possible to determine the duration ofthe initial off-set. The uncertainty in the duration of the initialoffset further increases the amount of error in the determination ofelapsed time within the sleep period resulting in an even greater riskthat the next paging slot will be missed. In an exemplary system whereinthe high frequency clock operates at 9.68 megahertz and the sleep clockoperates at 32 kilohertz, there are about 300 cycles of the highfrequency clock within each cycle of the sleep clock. Therefore, even ifthe system can reliably compensate for frequency drift, the highfrequency clock may still need to be activated as many as 300 cycles ofthe high frequency clock earlier than necessary to thereby account forthe unknown duration of the intial offset. Also, because there-activation of the high frequency clock at the end of the sleep periodis synchronized with transitions in the low frequency clock, the degreeof precision by which the high frequency clock can be re-activated islimited by the precision of the low frequency clock. For example, evenif the system reliably and precisely determines that the correctduration of the sleep period is 853.44 cycles of the sleep clock, thesystem will need to re-activate the high frequency clock no later thanthe detected transition of the 853rd cycle and therefore will notproperly account for the remaining fractional number of cycles, i.e. theremaining 0.44 cycles. With about 300 cycles of the high frequency clockoccurring within each cycle of the sleep clock, in the example the highfrequency clock is therefore turned on an additional 130 cycles earlierthan necessary. In another example, if the correct duration of the sleepperiod is 853.99 cycles of the sleep mode clock, the high frequencyclock will be turned on nearly 300 cycles earlier than necessary.

Hence, when using a low-frequency clock signal to track time during asleep period, the mobile station is usually configured to return to anactive mode well in advance of a next expected paging slot to therebyovercome possible timing errors cause by frequency drift in the lowfrequency clock and to compensate for the relative lack of precision inthe low frequency clock. For example, if paging slots occur every 26.67milliseconds, the mobile station may be programmed to activate the highfrequency clock after only, for example, 25 milliseconds of sleep toensure that the next paging slot is not missed. Hence, optimal powersavings are not achieved.

One technique that has been proposed for compensating for timing errorsinherent in low frequency, low power clock signal generators is to adapta length of each sleep period based upon a timing accuracy of a previoussleep period. More specifically, if the mobile telephone wakes up toolate within one sleep period to detect paging signals, the mobilestation is adjusted to wake up earlier in the next sleep period. Todetermine whether a sleep period is too long or too short, the mobilestation attempts to detect a unique word within a received pagingsignal, such as a message preamble which signifies the beginning of anassigned slot. If the unique word is not detected, the mobile stationconcludes that it woke up too late and therefore the sleep duration isdecreased for the next sleep period. If the unique word was properlyreceived, the mobile station either woke up on time or too early and thesleep duration is increased slightly for the next sleep period. Oneproblem with the aforementioned technique is that it assumes that anyfailure to detect the unique word is the result of a timing error.However, there may be other reasons besides the duration of the sleepperiod that the unique word was not correctly received and demodulated,such as poor communication channel quality conditions. Moreover, even iffailure to detect the unique word was the result of a timing errorrather than other communication errors, the system still does notcompensate for initial and final off-sets caused by the relative lack ofprecision in the low power, low frequency clock signal and thereforedoes not provide for optimal power savings.

A significant improvement is provided in U.S. patent application Ser.No. 09/134,808, entitled “Synchronization of a Low Power Oscillator witha Reference Oscillator in a Wireless Communication Device UtilizingSlotted Paging,” filed Aug. 14, 1998 and assigned to the assignee of thepresent invention. In the aforementioned patent application, timingerrors are corrected without relying upon the failure to receiveportions of transmitted signals. Rather, the system includes a frequencyerror estimation unit for directly estimating frequency drift in the lowpower, low frequency clock. In one example described in the patentapplication, frequency drift in the low frequency clock is determined bytiming the low frequency clock using a high frequency clock duringperiods of time when the high frequency clock is active. For example,during each paging slot when the high frequency clock of the mobilestation is operating, the frequency error in the low frequency clock iscalculated based upon the high frequency clock. The system operates tosynchronize the activation of the high frequency clock very precisely totransitions in the low frequency clock signal.

Although the system of the aforementioned patent application provides asignificant improvement over systems which rely on the detection ofunique words within signals transmitted to the mobile station,considerable room for improvement remains. For example, theaforementioned initial and final offsets are not taken into account.Accordingly, even with the improved system of the patent application,the high frequency clock, signal must usually be activated somewhat inadvance of the next expected paging slot to account for remaining timingerrors. Hence, optimal power savings are not achieved. It would bepreferable to provide a system wherein the active mode high frequencyclock is turned on as close as possible to the next paging slot topermit maximum power savings during the sleep period and it is to thatend that aspects of the invention are primarily directed. In particular,it is desirable to provide a system which compensates for theaforementioned initial and final offsets to re-activate the highfrequency clock to be re-activated based upon fractional portions of thelow frequency clock, and particular aspects of the invention aredirected to those ends.

SUMMARY OF THE INVENTION

In accordance with the invention, a device is provided for use inactivating an active-mode clock following a sleep period for use withina mobile station wherein selected components of the mobile stationoperate using a sleep-mode clock during the sleep period and a fasteractive-mode clock during non-sleep periods. The device includes awake-up estimation unit for estimating a wake up time using thesleep-mode clock. A compensation unit is provided for compensating fortiming off-set errors in the estimated wake up time caused bydifferences in precision between the sleep-mode clock and theactive-mode clock. An active-mode clock activation unit activates theactive-mode clock at the compensated wake-up time to terminate the sleepmode.

In an exemplary embodiment, the mobile station operates in a mobilecommunications system employing slotted paging. The device includes afrequency drift compensation unit for compensating for an error in theestimated wake up time caused by frequency drift in the sleep-modeclock. By compensating for both frequency drift and timing off-sets, theactive-mode clock is activated at a wake-up time closely insynchronization with a next paging slot and significant power savingsare achieved as compared with systems wherein the active-mode clock mustbe activated well in advance of the next paging slot to compensate forpossible timing errors.

In the exemplary embodiment, compensation for timing offsets andfrequency drift is achieved using a transition-mode clock which isemployed at both the beginning and the end of each sleep period. Thetransition-mode clock has a frequency substantially greater than that ofthe sleep mode clock. The transition-mode clock permits the device toconveniently compensate for both frequency drift errors and timingoffset errors to permit the active-mode clock to be re-activated laterin the sleep period. The transition-mode clock is deactivated shortlyafter the sleep period begins and is reactivated only slightly beforethe sleep period is due to end and hence very little power is consumedby the transition-mode clock. Moreover, because the transition-modeclock permits the components of the mobile station to be reactivatedlater within the sleep period, any increase in power required to operatethe transition-mode clock is more than compensated for by power servingsachieved by permitting a longer sleep period.

Method and apparatus embodiments of the invention are described.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout and wherein:

FIG. 1 is a block diagram illustrating a device, configured inaccordance with exemplary embodiment of the invention, for activating anactive-mode clock following a sleep period for use within a mobilestation of a mobile communication system employing slotted paging.

FIG. 2 is a block diagram illustrating the device of FIG. 1 in greaterdetail.

FIG. 3 is a timing diagram illustrating selected clock signalscontrolled by the device of FIGS. 1 and 2.

FIG. 4 is a flow chart illustrating a sequence of steps performed by thedevice of FIGS. 1 and 2 for activating the active-mode clock signalfollowing a sleep period.

FIG. 5 is a timing diagram of a specific implementtion of the device ofFIGS. 1 and 2 configured for use with CDMA.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the figures, exemplary embodiments of the invention willnow be described. The exemplary embodiments are first described withreference to the block diagrams of FIGS. 1 and 2 in combination with thetiming diagram of FIG. 3. The operation of the invention is thensummarized with reference to the flow chart of FIG. 4. Finally, aspecific implementation of the invention is described with reference toFIG. 5.

FIG. 1 illustrates an active-mode clock activation device 100 configuredfor activating a high frequency clock following a sleep period within amobile station (not shown) operating within a mobile communicationssystem employing slotted paging, such as a CDMA wirelesstelecommunications system. The high frequency clock is shown in FIG. 3and is identified by reference numeral 101. The sleep period duringwhich time the high frequency clock is deactivated is identified byreference numeral 103. The clock activation device is configured toactivate the high frequency clock as close as possible to a next pagingslot to maximize power savings during the sleep period while stillenabling the mobile station to wake up in time to receive and respond toany paging signals transmitted to the mobile station within the slot. Tothis end, clock activation device 100 of FIG. 1 includes components forcompensating for frequency drift within a low power, low frequency clockemployed during the sleep period and components for compensating forinitial and final timing offsets clock to thereby permit the highfrequency clock to be turned on with a high degree of precision despitethe fact that the sleep period is timed using only the low frequencyclock. In the following, the high frequency clock employed by the mobilestation during non-sleep modes is referred to as the active-mode clockand the low frequency clock signal employed for timing the sleep periodis referred to as a sleep-mode clock. The sleep-mode clock remainsactive at all times during both sleep periods and non-sleep periods.Compensation for frequency drift and for timing offset is performed, inpart, using a third clock having a frequency preferably equal to that ofthe active-mode clock. The third clock signal is referred to herein as atransition-mode clock and is employed at the beginning and end of eachsleep period. In FIG. 3, the transition-mode clock signal is identifiedby reference numeral 105 and the sleep-mode clock is identified byreference numeral 107. As can be seen, the transition-mode clock remainsactive for at least a few cycles following deactivation of theactive-mode clock and the transition-mode clock is reactivated at leasea few clock cycles prior to reactivation of the active-mode clock. Inthe following example, the active-mode clock and the transition-modeclock both operate at about 9.68 megahertz (mhz) and the sleep-modeclock operates at about 32 kilohertz (khz). However, different clockfrequencies may be employed in different implementations. In FIG. 1, theactive-mode, sleep-mode and transition-mode clock signals are generatedby an active mode clock generator 102, a sleep-mode generator clockgenerator 104, and a transition-mode clock generator 106, respectively.

Clock activation device 100 of FIG. 1 includes a wake up time estimationunit 108 for estimating the end of a sleep period based solely uponclock cycles counted using the sleep-mode clock, i.e. the number ofcycles of clock 107 of FIG. 3 counted during sleep period 103. Afrequency drift compensation unit 110 calculates an adjustment factorfor compensating for frequency drift within the sleep-mode clock andapplies the adjustment faster using transition-mode clock signal 105 ofFIG. 3. An offset time compensation unit 112 provides an additionaladjustment factor for adjusting the wake up time to compensate for anoffset between the beginning of the sleep period and a first countedclock cycle of the sleep-mode clock based upon clock cycles countedusing the transition-mode clock signal, i.e. the offset timecompensation unit compensates for the offset occurring between thebeginning of the sleep period 103 at FIG. 3 which commences at time 109and a first rising edge of sleep-mode clock 107 counted at time 111. Thewake up time estimate provided by unit 108 of FIG. 1 and the adjustmentfactors provided by units 110 and 112 are both forwarded to anactive-mode clock activation unit 114 which controls active-mode clockgenerator 102 to beginning outputting the active-mode clock signal foruse by other components of the mobile station such as a CDMA receiveunit used to receive paging signals within the paging slots transmittedfrom the base station. The active-mode clock activation unit may be,depending upon the particular implementation, controlled to activate theactive-mode clock sufficiently in advance of the next paging slot topermit warmup of components of the mobile station such as the CDMAreceive circuitry.

Thus, clock activation device 100 at FIG. 1 activates the active-modeclock following a sleep period based primarily upon the relativelyimprecise sleep-mode clock but adjusted based upon frequency drift andtiming offset adjustment factors using the much faster transition modeclock which is deactivated during most of the sleep period. In thismanner, considerable power savings may be achieved over previoussystems.

FIG. 2 provides details regarding the components of the wake up timeestimation unit, the frequency drift compensation unit and the offsettime compensation unit of FIG. 1. An exemplary sleep period of 26.67milliseconds will be used in the description, although other sleepperiod durations may be employed. A sleep controller 116 initiates thesleep period by deactivating active-mode clock generator 102 and bycontrolling estimation unit 108 and compensation units 110 and 112 tobegin operation. Wake up time estimation unit 108 provides an estimateof the end of the sleep period by counting the number of clock cycleswithin the sleep-mode clock (107 of FIG.2) and comparing the count tothe expected number of clock signals expected within the sleep periodassuming that the sleep-mode clock is not subject to any frequency driftand assuming that a first rising edge of the sleep-mode clock issynchronized with the start time of the sleep period. To this end,estimation unit 108 includes a memory register 118 storing the expectednumber of clock cycles of the sleep period minus any necessary warmupperiod and minus a maximum amount of time needed to adjust for offseterrors and frequency drift errors. The warmup period is a predeterminedvalue. The maximum amount of adjustment required to compensate for aninitial offset at the beginning of the sleep period is one and a half(1_) cycles the sleep mode clock. The maximum amount of time necessaryto adjust for frequency drift errors is a predetermined value calculatedbased upon maximum expected drift amounts based upon maximim expectedtemperature and power variations. These values may be calculated,without undue experimentation, based for example upon tests using samplecomponents subject to expected temperature variations. For the examplewherein the active-mode clock operates at 9.83 mhz and the sleep-modeclock operates at 30 khz, the memory register stores the number 300minus the warmup period and minus the maximum expected drift and offsetvalues. Thus, the memory register stores a number of clock cycles of thesleep-mode clock for a time period less than the actual expectedsleep-mode period by an amount sufficient to permit a warmup period andto permit the transition-mode clock to be used to more preciselycompensate for time variations.

A counter 120 is triggered by sleep mode controller 116 to begincounting rising edges of the sleep-mode clock signal. A controller 122receives the count and compares the count with the expected number ofclock cycles stored within memory register 118 and outputs an indicatorsignal when the clock equals the value in the memory register toindicate estimated completion of the sleep period. The indicator signalis transmitted to active-mode clock activation unit 114 for permittingreactivation of the active-mode clock subject to the adjustment factorsprovided by frequency drift compensation unit 110 and offset unit 112.Because the indicator signal is generated based upon the number ofcycles in the memory which take into account any warmup period and anymaximum expected adjustment period, the indicator signal therefore doesnot identify the actual expected end of the sleep period. Rather, theindicator signal is transmitted sufficiently in advance of the end ofthe expected sleep period to permit warmup and to permit the actual endof the sleep period to be precisely set using the adjustment factorsprovided by the frequency drift estimation unit and the offsetestimation unit.

Frequency drift unit 110 includes a frequency drift estimation unit 124for estimating an amount of drift in, for example, parts per millionwithin the sleep-mode clock as a result of temperature variations andthe like. In one example, the frequency drift estimation unit operatesonly while the active-mode clock generator is operating. The frequencydrift estimation unit receives both the sleep-mode clock signal and theactive-mode clock signal and counts the number of active-mode clockcycles within each cycle of the sleep-mode clock to maintain a runningaverage of the actual frequency of the sleep-mode clock. To this end, amoving average window (MAW) filter may be employed. The frequency driftestimation unit compares an expected number of active-mode clock cyclesfound within each sleep-mode cycle with the actual number counted andcalculates a frequency drift factor accordingly. In the example whereinthe active-mode clock operates at 9.83 mhz and the sleep-mode clockoperates at 30 khz, the frequency drift estimation unit is configured toexpect about 300 cycles of the active-mode clock within each cycle ofthe sleep-mode clock.

In a preferred implementation, however, the frequency drift estimationunit is configured as described in a U.S. Patent Application entitled“Method and Apparatus for Compensating for Frequency Drift in a LowFrequency Clock Within a Mobile Station Operating in a Slotted PagingMode”, filed contemporaneously herewith and assigned to the assignee ofthe present invention. This application is incorporated by referenceherein.

The estimated frequency drift value generated by estimation unit 124 isoutput to an expected timing error calculation unit 126 which calculatesa timing error in milliseconds expected to occur between the wake uptime generated by wake up estimation unit 108 and the next paging slot.For example, if the output of the frequency drift estimation unit is aexpressed in parts per million, then the expected timing errorcalculation unit converts the parts per million value to an actual timevalue based upon the known length of the sleep period. The expectedtiming error is transmitted to a timing error adjustment unit 128 whichcontrols active-mode clock activation unit 114 to delay activation ofthe active-mode clock by an amount sufficient to compensate for theestimated timing error has elapsed. To this end, the timing erroradjustment unit includes a transition mode clock activation unit 132 foractivating the transition mode clock generator 106 upon receipt of theindicator signal provided by the wake up estimation unit 108. A counter134 then begins counting a number of clock cycles within the transitionmode clock, i.e. the counter counts cycles of transition-mode clock 105commencing at time 113 as shown in FIG. 3. A controller 136 calculatesthe number of transition mode clock cycles within the expected timingerror, then outputs a control signal to the active-mode clock activationunit when the count maintained by counter 134 is equal to the calculatednumber of transition mode clock cycles, i.e. the control signal isoutput at time 115 as shown in FIG. 3. Active-mode clock activation unit114 is configured to active the active-mode clock only where an enablingcontrol signal is received from the frequency drift compensation unitand the off-set time compensation unit. Hence, the active mode clock isdelayed at least until the control signal output by controller 136 ofthe frequency drift compensation unit is received.

Offset compensation unit 112 includes an initial offset time estimationunit 138 and an offset error adjustment unit 140 which together operateto calculate an offset between the beginning of the sleep period and thefirst rising edge of the sleep-mode clock and to output a control signalto the active-mode clock activation unit to adjust the active mode clockactivation accordingly. In the example of FIG. 3, the transition-modeclock is activated sometime prior to the initiation of the sleep periodand is therefore operational at the beginning of the sleep period. Acounter 142, activated by sleep controller 116, begins counting cyclesof the transition mode clock at the beginning of the sleep period.Counter 142 continues to count cycles until a first rising edge of thesleep-mode clock is detected by a detection unit 144, i.e. the counter142 counts cycles of transition-mode clock 105 occurring between time109 and time 111 as shown in FIG. 3. The number of counts is storedwithin a memory register 146 and the transition mode clock is thendeactivated by a transition clock deactivation unit 148. Thetransition-mode clock remains inactive throughout the remaining portionsof the sleep period until reactivated by a reactivation unit 150. Thereactivation unit activates the transition mode clock upon receiving theindicator signal from wake up estimation unit 108. At that time, acontroller 152 reads the value stored within memory register 146 andactivates a counter 154 for counting the number of counts that had beenstored in register 146. Upon expiration of counter 154, a control signalis sent from controller unit 152 to active-mode clock activation unit114 to enable activation of the active-mode clock.

Thus, wake up time estimation unit 108 generates an indicator signal ata predetermined amount of time prior to the end of the sleep period asestimated using only the sleep-mode clock generator. The frequency driftcompensation unit and the offset compensation unit delay activation ofthe active-mode clock from the receipt of the indicator signal toaccount for the duration of the warmup period and to compensation forfrequency drift within the sleep-mode clock and to compensate for anyoffset between the beginning of the sleep period and a first countedcycle of the sleep-mode clock. In this manner, the active mode clock issynchronized to resume operation at the beginning of the warmup periodgiving it just sufficient enough time to allow components of the mobilestation to warm up prior to the next paging slot. In this manner, powersavings are achieved over devices which either do not compensate forfrequency drift or do not compensate for the initial offset period.

Thus, FIGS. 1-3 collectively illustrate an exemplary embodiment of theinvention operative to reactivate a high frequency clock signal at theend of a sleep period while compensating for both frequency drift andinitial timing offsets, in part, using a transition-mode clock signal atthe beginning and ends of the sleep periods. The method employing thetransition-mode clock signal will now be briefly summarized withreference to the flow chart of FIG. 4. Initially, at step 200, themobile station activates the transition-mode clock prior to the of asleep period. At step 202, the high frequency active-mode clock isdeactivated thereby initiating the sleep period. During step 204, thedevice counts a number of rising edges in the transition-mode signaloccurring from the point and time the active-mode clock is terminateduntil a next rising edge of the sleep-mode clock, which remains activeat all times. The number of counts determined during step 204 is storedin a counter. At step 204, the transition-mode clock is deactivated.During step 208, an estimate of the amount of elapsed time during thesleep period is calculated using only the low frequency sleep-modeclock. At step 210, the transition-mode clock signal is reactivated. Thereactivation time is based upon the estimated end of the sleep period asdetermined using the sleep-mode clock minus a predetermined amount oftime sufficient to allow for compensation of maximum expected frequencydrift and timing off-set errors. During step 212 the number of cyclesoccurring in the transition-mode clock signal is counted until the countequals the number of counts recorded following step 204 plus the numberof counts necessary to compensate for the calculated frequency drift.Once the appropriate number of cycles of the transition-mode clock haveelapsed, then at step 214, the high frequency active-mode clock isreactivated, permitting components of the mobile station to warm up intime to receive a paging signal transmitting in the next paging slot.

In the following, an implementation of the invention configured for usewith CDMA systems conforming to the IS-95A standard promogated by theTelecommunications Industry Association (TIA) is briefly described.According to the IS-95A standard, a CDMA “subscriber station” operatingin the slotted mode maximizes the standby time by going to sleep, basedon a parameter, Slot Cycle Index (SCI). The subscriber station wakes upevery (1.28*2SCI) seconds to monitor an assigned 80 ms slot to receivepages. For example with SCI=0, the subscriber station ideally remainsawake for 80 ms and sleeps for 1.2 sec. In practice, it needs to wake upa sufficient amount of time ahead of the next slot boundary to take careof events such as RF warm-up, synthesizer stabilization, clock settling,CDMA pilot search and acquisition, finger reassignment and decoderwarm-up.

In each sleep cycle, the unit sleeps in “catnaps” to allow a goodresponse time if the user presses a key while the subscriber station isasleep. The sleep cycle length and catnap length are chosen to bemultiples of a psuedo-number (PN) roll (e.g. 26.67 ms) so that uponwake-up, a search may be made at the station find the pilot atapproximately the same position. Each catnap is further divided into:(1) “sleep time,” when the entire unit is put to sleep and (2) “warm-uptime,” when the RF and analog units are turned on for warm-up. When thesubscriber station is asleep, the system time is approximatelymaintained by clocking the counters that keep track of sleep durationwith a combination of a Slow Clock (SC) for coarse timing (maximumresolution of 1/60 k=16.7 sec) and a SLPCHIPX8 clock for fine timing(resolution of 1/(8*1.2288e6)=0.102 sec).

An example of the events that constitute a sleep cycle is shown in FIG.5. Waveform E marks each event in the sleep cycle as described below:

Before t1: When it is time to sleep, software of the subscriber stationshuts off all unnecessary clock regimes except for a CDMA demodulatorand decoder clock regime, RXCHIPX8 (Waveform B), which is based on theactive-mode clock CHIPX8.

A multiple of 26.67 ms is split into sleep time and warm-up time andprogrammed as the duration of a first catnap interval throughSLEEP_INTERVAL and WU_TIME registers.

Software running on a microprocessor of the subscriber station writes anASIC_SLEEP_ARM bit of a SLEEP_CTL register, indicating that thesubscriber station can go to sleep on the next PN roll (indicated byt1).

All along, the sleep clock (SC) (Waveform D) is run asynchronous to theCDMA clock regime CHIPX8, while the SLPCHIPX8 (Waveform C), i.e. thetransition-mode clock is in sync with the RXCHIPX8, having been derivedfrom the same source, CHIPX8.

At time t1 when the next PN roll occurs, the RXCHIPX8 clock regime isdisabled putting the subscriber station to sleep. From this pointonwards, the sleep period should be very close to multiples of 26.67 msas determined using counters SLEEP_INTERVAL and WU_TIME running off theSC. To account for the asynchronous SC, a counter calledCHIPX8_SLEEP_TIME starts counting the SLPCHIPX8's that have elapsed fromt1 to the next rising edge of the SC.

At time t2, the rising edge of SC occurs at which time theSLPCHIPX8clock regime is disabled, freezing the CHIPX8_SLEEP_TIME,thereby providing an estimate of the time duration (t2−t1) in chipx8units.

At time t3, after half an SC duration, a SLEEP_N signal (Waveform A)goes low on the falling edge of the SC. This puts the other digital,analog and RF components in the subscriber station in a low power mode.If there are NSC chipx8's in a SC, the total time elapsed at this pointof time is given by: T_(A)=(t2−t1)+(t3−t2)={CHIPX8_SLEEP_TIME+_NSC}chipx8s. It may be noted that from this definition, T_(A) will be in therange of_(—)−1_slow clock cycles. Subsequent catnaps should be adjustedto account for this extra time slept owing to the asynchronous sleepcrystal. Also the counter SLEEP_INTERVAL running off the SC starts tocounts down.

At time t4, the counter SLEEP_INTERVAL asserts the wake-up interruptwhen it reaches a zero count. The microprocessor of the subscriberstation wakes up and determines if hardware needs to be awake at thenext slot or to service a key-press event.

If neither of these conditions is met, the software ensures that thehardware can remain asleep by keeping the SLEEP_N signal active duringthe warm-up count down (via the WU_TIME counter). At this time, thesoftware estimates the number of SC's needed to sleep in the next catnapbased on several factors such as next catnap length, asynchronous lag inthe slow clock, drift and truncation errors that arise from the use ofSC to approximate a multiple of PN roll.

At time t5, when the WU_TIME counter expires, a new value obtained inthe previous step is loaded into the SLEEP_INTERVAL counter. The WU_TIMEcounter is a precomputed constant specified by the RF hardware warm-uprequirements. The microprocessor goes back to sleep awaiting the wake-upinterrupt from the next catnap.

At time t6, if however there are any pending interrupts to be servicedor if this is the last catnap allowed in this sleep cycle, the hardwareis woken up to be ready for the next slot by causing the SLEEP_N signalto go inactive at the wake-up interrupt. While the WU_TIME countercounts down, the analog and RF components warm up.

At time t7, the WU_TIME counter expires indicating the end of the lastcatnap and the SLPCHIPX8 regime is turned on at time t8. As a side note,the total time elapsed during all the catnaps, denoted by T_(B)=t7−t3,will be close to integer multiple of SC's. Due to the several factorsmentioned previously that go into the sleep calibration, there willusually be a residual amount of time (a fraction of SC) for which thesubscriber station needs to still remain asleep. This fractional SC(denoted by T_(C)) is converted into chipx8 units and programmed intothe CHIPX8_SLEEP_TIME that starts counting down clocked by theSLPCHIPX8.

At time t9, the CHIPX8_SLEEP_TIME expires, and the hardware turns on theRXCHIPX8 at time t10. The last time duration of interest is T_(C)=t9−t7.

The exemplary embodiments have been primarily described with referenceto schematic diagrams illustrating pertinent features of theembodiments. It should be appreciated that not all components of acomplete implementation of a practical system are necessarilyillustrated or described in detail. Rather, only those componentsnecessary for a thorough understanding of the invention have beenillustrated and described. Actual implementations may contain morecomponents or, depending upon the implementation, fewer components. Thedescription of the exemplary embodiments is provided to enable anyperson skilled in the art to make or use the present invention. Variousmodifications to these embodiments will be readily apparent to thoseskilled in the art and the generic principles defined herein may beapplied to other embodiments without the use of the inventive faculty.Thus, the invention is not intended to be limited to the embodimentsshown herein but is to be accorded the widest scope consistent with theprinciples and novel features disclosed herein.

What is claimed is:
 1. A device for activating an active-mode clockfollowing a sleep period for use within a mobile station whereinselected components of the mobile station operate using a sleep-modeclock during the sleep period and a faster active-mode clock duringnon-sleep periods said device comprising: means for estimating a wake uptime using the sleep-mode clock; means for compensating for errors inthe estimated wake up time caused by differences in precision betweenthe sleep-mode clock and the active-mode clock; and means for activatingthe active-mode clock at the compensated wakeup time, wherein the meansfor compensating for errors in the estimated wake up time caused bydifferences in precision between the sleep-mode clock and theactive-mode clock includes: means for determining the duration of aninitial off-set occurring between a beginning of the sleep period and afirst counted cycle of the sleep-mode clock and for adjusting theestimated wake-up time based on the duration of the initial off-set; andmeans for determining the duration of a final off-set occurring betweena final counted cycle of the sleep-mode clock and an end of the sleepperiod and for adjusting the estimated wake-up time based on theduration of the final off-set.
 2. The device of claim 1 furtherincluding means for compensating for an error in the estimated wake uptime caused by frequency drift in the sleep-mode clock.
 3. The device ofclaim 1 wherein said means for compensating for an error in theestimated wake up time caused by frequency drift in the sleep-mode clockcomprises: means for estimating the frequency drift occurring in thesleep-mode clock occurring during a single sleep period; and means forcalculating an expected timing error between an actual sleep periodduration and a duration of a sleep period as counted by a sleep-modeclock subject to the estimated frequency drift; timing error adjustmentmeans for controlling the means for activating the active-mode clock toadjust the activation time based the expected timing error.
 4. Thedevice of claim 3 wherein the timing error adjustment means comprises:means for activating an transition-mode clock signal at a start timeprior to the estimated wake-up time, said transition-mode clock having afrequency substantially greater than that of the sleep-mode clock; meansfor calculating a number of clock cycles of the transition-mode clocksignal occurring between the transition-mode clock start time and theexpected end of the sleep period; means for calculating a number ofclock cycles of the transition-mode clock signal occurring within theexpected timing error; means for adding the number of clock cyclesoccurring between the transition-mode clock start time and the expectedend of the sleep period and the number of clock cycles occurring withinthe expected timing error to yield a combined number of clock cycles;means for counting an elapsed number of cycles in the transition-modeclock; and means for controlling the means for activating theactive-mode clock to delay activation of the active-mode clock followingthe transition-mode clock start time until the combined number of clockcycles of the transition-mode clock have elapsed.
 5. The device of claim1 wherein said means for determining the duration of an initial off-setoccurring between a beginning of the sleep period and a first countedcycle of the sleep-mode clock and for adjusting the estimated wake-uptime based on the duration of the initial off-set comprises: means forgenerating an transition-mode clock signal having a frequencysubstantially greater than that of the sleep-mode clock; means forcounting an elapsed number of cycles in the transition-mode clockoccurring between a start of the sleep period and a next rising edge ofthe sleep-mode clock; means for storing the counted number of clockcycles; means for de-activating the transition-mode clock; means forre-activating the transition-mode clock signal at a start time prior tothe estimated wake-up time; and delay means for controlling the meansfor activating the active-mode clock to delay activation of theactive-mode clock until the counted number of clock cycles of thetransition-mode clock have elapsed.
 6. The device of claim 1 wherein themobile station operates in a mobile communications system employingslotted paging and wherein the sleep period is substantially equal to atime period between consecutive paging slots such that the active-modeclock is activated at a wake up time substantially synchronized with anext paging slot.
 7. A device for activating an active-mode clockfollowing a sleep period for use within a mobile station whereinselected components of the mobile station operate using a sleep-modeclock during the sleep period and the faster active-mode clock duringnon-sleep periods, said device comprising: a wake-up estimation unit forestimating a wake up time using the sleep-mode clock; a compensationunit for compensating for errors in the estimated wake up time caused bydifferences in precision between the sleep-mode clock and theactive-mode clock; and an active-mode clock activation unit foractivating the active-mode clock at the compensated wake-up time,wherein the compensating unit includes: an initial off-set compensationunit for determining the duration of an initial off-set occurringbetween a beginning of the sleep period and a first counted cycle of thesleep-mode clock and for adjusting the estimated wake-up time based onthe duration of the initial off-set; and a final off-set compensationunit for determining the duration of a final off-set occurring between afinal counted cycle of the sleep-mode clock and an end of the sleepperiod and for adjusting the estimated wake-up time based on theduration of the final off-set.
 8. The device of claim 7 furtherincluding a frequency drift compensation unit for compensating for anerror in the estimated wake up time caused by frequency drift in thesleep-mode clock.
 9. The device of claim 7 wherein said wake-upestimation unit comprises a register for storing a predetermined numberof clock cycles occurring within a single sleep period for a clockoperating at a predetermined intended sleep-mode frequency; a counterfor counting an elapsed number of cycles in the sleep-mode clock duringthe sleep period; and an indicator unit for indicating when the countednumber of clock cycles equals the expected number of clock cycles. 10.The device of claim 7 wherein said frequency error compensation unitcomprises: a frequency drift estimation unit for estimating thefrequency drift occurring in the sleep-mode clock occurring during asingle sleep period; an expected timing error calculation unit forcalculating an expected timing error between an actual sleep periodduration and a duration of a sleep period as counted by a sleep-modeclock subject to the estimated frequency drift; and a timing erroradjustment unit for controlling the active-mode clock activation unit toadjust the activation time based the expected timing error.
 11. Thedevice of claim 10 wherein the timing error adjustment unit comprises:an transition-mode clock activation unit for activating antransition-mode clock signal at a start time prior to the estimatedwake-up time, said transition-mode clock having a frequencysubstantially greater than that of the sleep-mode clock; a firstcalculation unit for calculating a number of clock cycles of thetransition-mode clock signal occurring between the transition-mode clockstart time and the expected end of the sleep period; a secondcalculation unit for calculating a number of clock cycles of thetransition-mode clock signal occurring within the expected timing error;an adder for adding the number of clock cycles occurring between thetransition-mode clock start time and the expected end of the sleepperiod and the number of clock cycles occurring within the expectedtiming error to yield a combined number of clock cycles; a counter forcounting an elapsed number of cycles in the transition-mode clock; and acontroller for controlling the active-mode clock activation unit todelay activation of the active-mode clock following the transition-modeclock start time until the combined number of clock cycles of thetransition-mode clock have elapsed.
 12. The device of claim 7 whereinsaid initial off-set compensation unit for determining the duration ofan initial off-set occurring between a beginning of the sleep period anda first counted cycle of the sleep-mode clock and for adjusting theestimated wake-up time based on the duration of the initial off-setcomprises: a clock signal generator for generating an transition-modeclock signal having a frequency substantially greater than that of thesleep-mode clock; a counter for counting an elapsed number of cycles inthe transition-mode clock occurring between a start of the sleep periodand a next rising edge of the sleep-mode clock; a register for storingthe counted number of clock cycles; and a de-activation unit fordeactivating the transition-mode clock; and wherein said off-set erroradjustment unit comprises a re-activation unit for re-activating thetransition-mode clock signal at a start time prior to the estimatedwake-up time; and a delay unit for controlling the active-mode clockactivation unit to delay activation of the active-mode clock until thecounted number of clock cycles of the transition-mode clock haveelapsed.
 13. The device of claim 7 wherein the mobile station operatesin a mobile communications system employing slotted paging and whereinthe sleep period is substantially equal to a time period betweenconsecutive paging slots such that the active-mode clock is activated ata wake up time substantially synchronized with a next paging slot.
 14. Amethod for activating an active-mode clock following a sleep period foruse within a mobile station wherein selected components of the mobilestation operate using a sleep-mode clock during the sleep period and thefaster active-mode clock during non-sleep periods, said methodcomprising the steps of: estimating a wake up time using the sleep-modeclock; compensating for errors in the estimated wake up time caused bydifferences in precision between the sleep-mode clock and theactive-mode clock; and activating the active-mode clock at thecompensated wake-up time, wherein the step of compensating for errors inthe estimated wake up time caused by differences in precision betweenthe sleep-mode clock and the active-mode clock includes the steps of:determining the duration of an initial off-set occurring between abeginning of the sleep period and a first counted cycle of thesleep-mode clock and for adjusting the estimated wake-up time based onthe duration of the initial off-set; and determining the duration of afinal off-set occurring between a final counted cycle of the sleep-modeclock and an end of the sleep period and for adjusting the estimatedwake-up time based on the duration of the final off-set.
 15. The methodof claim 14 further including the step of compensating for an error inthe estimated wake up time caused by frequency drift in the sleep-modeclock.
 16. The method of claim 14 wherein said step of estimating thewake up time comprises the steps of: storing a predetermined number ofclock cycles occurring within a single sleep period for a clockoperating at a predetermined intended sleep-mode frequency; counting anelapsed number of cycles in the sleep-mode clock during the sleepperiod; and indicating when the counted number of clock cycles equalsthe expected number of clock cycles.
 17. The method of claim 14 whereinsaid step of compensating for an error in the estimated wake up timecaused by frequency drift in the sleep-mode clock comprises the stepsof: estimating the frequency drift occurring in the sleep-mode clockoccurring during a single sleep period; calculating an expected timingerror between an actual sleep period duration and a duration of a sleepperiod as counted by a sleep-mode clock subject to the estimatedfrequency drift; and adjusting the activating time of the active-modeclock based the expected timing error.
 18. The method of claim 14wherein the step adjusting the activating time comprises the steps of:activating an transition-mode clock signal at a start time prior to theestimated wake-up time, said transition-mode clock having a frequencysubstantially greater than that of the sleep-mode clock; calculating anumber of clock cycles of the transition-mode clock signal occurringbetween the transition-mode clock start time and the expected end of thesleep period; calculating a number of clock cycles of thetransition-mode clock signal occurring within the expected timing error;adding the number of clock cycles occurring between the transition-modeclock start time and the expected end of the sleep period and the numberof clock cycles occurring within the expected timing error to yield acombined number of clock cycles; counting an elapsed number of cycles inthe transition-mode clock; and delaying activation of the active-modeclock following the transition-mode clock start time until the combinednumber of clock cycles of the transition-mode clock have elapsed. 19.The method of claim 14 wherein said step of compensating for an error inthe estimated wake up time caused by an initial off-set comprises:generating an transition-mode clock signal having a frequencysubstantially greater than that of the sleep-mode clock; counting anelapsed number of cycles in the transition-mode clock occurring betweena start of the sleep period and a next rising edge of the sleep-modeclock; storing the counted number of clock cycles; and de-activating thetransition-mode clock; and wherein said step of activating theactive-mode clock comprises the steps of re-activating thetransition-mode clock signal at a start time prior to the estimatedwake-up time; and delaying activation of the active-mode clock until thecounted number of clock cycles of the transition-mode clock haveelapsed.
 20. The method of claim 14 wherein the mobile station operatesin a mobile communications system employing slotted paging and whereinthe sleep period is substantially equal to a time period betweenconsecutive paging slots such that the active-mode clock is activated ata wake up time substantially synchronized with a next paging slot.